Gold-tin solder suitable for self-aligning applications

Abstract

A tin-rich gold-tin solder is disclosed which is particularly advantageous for self-aligning applications. When utilized with gold-plated bond locations, the out-diffusion of tin from the solder during heating functions to shift the composition of the remaining solder closer to the eutectic value, thus preserving the liquid state of the solder and improving its reflow quality with respect to conventional eutectic solders.

Description

BACKGROUND OF THE INVENTION1. Technical FieldThe present invention relates to a gold-tin solder and, more particularly, to a tin-rich, gold-tin solder.2. Description of the Prior ArtSoldering is a well-known technique for securing metal piece parts in a permanent fashion. Solder material must have the capacity to form metallurgical bonds with the two base metals that are to be joined. The bonding process results in the formation of an alloy in the surface of the base metal characterized by atoms of the soldering composition interspersed between atoms of the base metal.When the solder is heated to a molten state, it exists as a round droplet as a result of the attraction between the molecules forming the alloy. This attraction is commonly referred to as surface tension. For many technologically advanced applications, such as semiconductor and lightwave technologies, a solder composition of choice is a eutectic gold-tin material which is approximately 80% by weight gold and 20% by wei...

AI Technical Summary

Benefits of technology

Further, the soldering process of the present invention may be optimized by heating the solder preform within a reducing ambient such that the further growth of any tin oxide on the surface of the preform (skin oxide) is significantly reduced.

An advantage of the solder composition of the present invention is that self-alignment of piece parts may be achieved. In particular, the utilization of a tin-rich solder allows for the solder to remain liquid and completely cover (i.e., "wet") both bonding surfaces. Therefore, by controlling the size and location of the bonding surfaces, the liquification of the tin-rich solder will result, through surface tension effects, in registration (i.e., centering) of the device with respect to the underlying bond pad.

Problems solved by technology

There are many known disadvantages to this procedure.

First, the reproducibility of the reflow is questionable and sensitive to process variations, particularly since commonly available eutectic Au / Sn solder may only be guaranteed .+-.1% around the eutectic 80 / 20 composition.

Additionally, the solid state diffusion of tin into an underlying gold bond pad will shift the preform composition to the gold-rich side of the eutectic and could result in poor reflow by raising the melting temperature of the resulting alloy.

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